1. Field of the Invention
The present invention relates to printer heads, particularly those having an optical modulator element such as a liquid-crystal-optical shutter, and more particularly, to those having a liquid-crystal optical shutter which are applicable to printers utilizing an electrophotographic technique.
2. Description of the Prior Art
Recently there has been a growing demand for a printer which fulfills such requirements as high-speed and high-density printing, low price, low noise operation, and high reliability. This is due to a need for an output mechanism adaptable for recently developing electronic techniques in the field of information transmission, processing, and making. Almost all of the signals for information output are electric or are convertible into electric signals. As electric information output terminals for printers, there are used ink-jet printers, multi-stylus wire dot printers, laser beam printers, etc. Printers utilizing an electrophotographic copying system having a photosensitive drum are also widely used. Laser beam printers, however, have drawbacks in that these are expensive and require strict mechanical and optical accuracy in making them. Possible substitutes for laser beam printers include solid optical shutter arrays, but no optical shutter array satisfying various requirements has yet been put to practical use. There are a number of candidates for electro-optical elements, materials, or assemblies as a technique for realizing a successful solid shutter array. Among them, a system utilizing a liquid crystal has been noticed from early time in view of its easy production and its possibility of achieving optical modulation with low voltage and low power. However, liquid crystals have been deserted by many researchers as unpromising for substituting for the laser beam means, in that liquid crystals exhibit low response speeds and difficult time-division drive. Nevertheless, many efforts have been made over the years to operate liquid crystals at high response speeds, but with the result that none of the proposals hitherto made could accomplish a satisfactory technique superior to the laser beam technique. The main reasons for this are the insufficient understanding of the electrophotographic phenomenon of liquid crystals and the above-mentioned preconception that liquid crystals are not adapted to high speed operation or high density recording.
Some prior art suggested the application of a liquid-crystal-optical shutter array to the head of an electrographic copying machine for realizing a high response speed with a liquid crystal (e.g. Japanese Patent Laid-open Application Nos. 63507/82 and 63508/82). The liquid-crystal-optical shutter array has the form of a twisted nematic liquid crystal cell sandwiched between a pair of polarizing plates disposed usually in the cross state of nicol, and operates as follows: when no operational voltage is applied to the liquid crystal cell, the polarizing plate on the incident light side transmits only one of the P-component (P-polarized light) and S-component (S-polarized light) of the incident light and the polarizing plate on the other side (in the cross nicol state) shuts the above transmitted light, so that the array thus produces the closed state of the aperture. On the other hand, when operational voltage is applied to the cell, a change occurs in the operation mode of the liquid crystal and the one of P- and S-components of the incident light which is transmitted by the liquid crystal cell also passes the polarizing plate disposed behind the cell, so that the array produces the open state of the aperture.
However, the printer head utilizing such an optical modulator element as the liquid-crystal-optical shutter in the prior art has the drawback of a large loss of light intensity caused by the pair of polarizing plates used and hence a decreased quantity of light illuminating the photosensitive drum in the open state of the aperture. In a special case, for instance, the light quantity is decreased to about 1/2 by the polarizing plate disposed on the incident light side and is further decreased by the polarizing plate disposed behind the liquid crystal cell; thus the light quantity illuminating the photosensitive drum is decreased to about 25-50% of the light quantity emitted from the light source. A large loss of the light necessitates an increased light source wattage, and this produces undersirable effects such as excess heat generation and a change in temperature-dependent characteristics of the liquid crystal itself and results in an uneconomically large power consumption. With the conventional voltage to the light source, the printing speed is low because of a limited exposure of the photosensitive drum.